CMOS image sensors are increasingly being used as low cost imaging devices. A CMOS image sensor circuit includes a focal plane array of pixel cells, each one of the cells includes a photogate, photoconductor, or photodiode having an associated charge accumulation region within a substrate for accumulating photo-generated charge. Each pixel cell may include a transistor for transferring charge from the charge accumulation region to a sensing node, and a transistor, for resetting the sensing node to a predetermined charge level prior to charge transference. The pixel cell may also include a source follower transistor for receiving and amplifying charge from the sensing node and an access transistor for controlling the readout of the cell contents from the source follower transistor.
In a CMOS image sensor, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the sensing node accompanied by charge amplification; (4) resetting the sensing node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge from the sensing node.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994). See also U.S. Pat. Nos. 6,177,333 and 6,204,524, which describe the operation of conventional CMOS image sensors and are assigned to Micron Technology, Inc., the contents of which are incorporated herein by reference.
A schematic diagram of a conventional CMOS pixel cell 10 is shown in FIG. 1. The illustrated CMOS pixel cell 10 is a four transistor (4T) cell. The CMOS pixel cell 10 generally comprises a photo-conversion device 23 for generating and collecting charge generated by light incident on the pixel cell 10, and a transfer transistor 17 for transferring photoelectric charges from the photo-conversion device 23 to a sensing node, typically a floating diffusion region 5. The floating diffusion region 5 is electrically connected to the gate of an output source follower transistor 19. The pixel cell 10 also includes a reset transistor 18 for resetting the floating diffusion region 5 to a predetermined voltage; and a row select transistor 16 for outputting a signal from the source follower transistor 19 to an output terminal in response to an address signal.
FIG. 2 is a cross-sectional view of the pixel cell 10 of FIG. 1 depicting the photo-conversion device 23. The exemplary CMOS pixel cell 10 has a photo-conversion device 23, which may be formed as a pinned photodiode. The photodiode 23 has a p-n-p construction comprising a p-type surface layer 22 and an n-type photodiode region 21 within a p-type active layer 11. The photodiode 23 is adjacent to and partially underneath the transfer transistor 17. The reset transistor 18 is on a side of the transfer transistor 17 opposite the photodiode 23. As shown in FIG. 2, the reset transistor 18 includes a source/drain region 2. The floating diffusion region 5 is between the transfer and reset transistors 17, 18.
In the CMOS pixel cell 10 depicted in FIGS. 1 and 2, electrons are generated by light incident on the photo-conversion device 23 and are stored in the n-type photodiode region 21. These charges are transferred to the floating diffusion region 5 by the transfer transistor 17 when the transfer transistor 17 is activated. The source follower transistor 19 produces an output signal from the transferred charges. A maximum output signal is proportional to the number of electrons extracted from the n-type photodiode region 21.
Conventionally, a shallow trench isolation (STI) region 3 adjacent to the charge collection region 21 is used to isolate the pixel cell 10 from other pixel cells and devices of the image sensor. The STI region 3 is typically formed using a conventional STI process. The STI region 3 is typically lined with an oxide liner 38 and filled with a dielectric material 37. Also, the STI region 3 can include a nitride liner 39 between the oxide liner 38 and the dielectric material 37.
The nitride liner 39 provides several benefits, including improved corner rounding near the STI region 3 corners. The nitride liner 39 typically has tensile stress. Since the dielectric material 37 typically introduces compressive stress, the presence of the nitride liner 39 results in an overall decrease in stress levels. Reduced stress levels provide decreased leakage currents in the photo-conversion device 23. Particularly, leakage from trap-assisted tunneling and trap-to-trap tunneling mechanisms is decreased.
A common problem associated with the above described STI region 3 is dangling bonds (e.g., dangling Si- bonds) at the surface of the substrate 11 along the trench bottom 8 and sidewalls 9. The dangling bonds create a high density of trap sites along the trench bottom 8 and sidewalls 9. These trap sites are normally uncharged but become charged when electrons and holes become trapped in the sites. As a result of these trap sites formed along the bottom 8 and sidewalls 9 of the STI region 3, current generation near and along the trench bottom 8 and sidewalls 9 can be significant. Current generated from trap sites inside or near the photodiode 23 depletion region causes undesired dark current and an increase in fixed pattern noise.
Conventionally, hydrogen passivation is used to reduce the dangling bonds. However, the nitride liner 39 acts as a diffusion barrier for hydrogen (H2) during passivation and reduces passivation of the dangling bonds. Therefore, when the nitride liner 39 is used dark current may increase.
It is desirable to have an isolation region with a nitride liner and reduced dangling bonds.